ACT is not backing down on a campaign against the Emissions Trading Scheme, saying it will gobble up any benefit from tax cuts.
ACT leader Rodney Hide says the party remains fiercely opposed to the scheme.
Tensions between National and its support party have become apparent after National Party ministers accused ACT of talking up the potential costs of the ETS.
Agriculture Minister David Carter sent a memo to the National caucus saying that ACT and Federated Farmers are running a concerted campaign against the scheme. He said the figures being used were, at best, misleading.
Mr Carter says the estimated cost for a dairy farm in the scheme's first year is $3900.
ACT says that is the figure it has been using, but the cost will rise to $10,000 per year by 2015.
Climate Change Minister Nick Smith says ACT is taking advantage of the fact that the issue is complex.